The invention concerns an ATR (attenuated total reflection) objective for an IR (infrared) microscope.
An IR microscope of this type comprising an ATR crystal is disclosed e.g. in U.S. Pat. No. 5,581,085.
Infrared (IR) spectroscopy is used to obtain analytical information about a sample. The chemical bonds in the sample absorb or reflect IR light in dependence on the wavelength of the IR light.
For samples with insufficient reflectivity and insufficient transparency, the IR spectroscopic investigation by means of attenuated total reflection (ATR) may sometimes also be in the form of an internal reflection. With attenuated total reflection, IR radiation is guided through a crystal with a high refractive index in the IR range (e.g. of germanium), which contacts the surface of the sample. The IR radiation marginally enters the sample, thereby producing sufficient interaction for a spectroscopic analysis, and is subsequently guided back through the crystal and detected by an IR detector. A good ATR measurement requires intimate contact between the ATR crystal and the sample, for which purpose the ATR crystal is pressed onto the sample, in most cases using a spring. Different contact pressures are used for different sample types (in particular sample hardnesses).
An IR microscope obtains analytical information specifically for a certain location or a certain area of the sample. An IR microscope advantageously has both an optical path for visible light and an optical path for IR light. Due to the largely common optical path of the visible light and the IR light, one can view with the visible light the location or area of the sample from which the analytical information is obtained by the IR light.
In an IR microscope with an ATR objective in the optical path, the ATR crystal fundamentally obstructs observation of the sample with visible light. For this reason, the ATR crystal must be removed from the optical path in the optical observation mode. Towards this end, the ATR crystal according to prior art is manually pivoted to the side or manually removed from the sample position in an upward direction. According to the “HYPERION Benutzerhandbuch” (HYPERION manual) of the company Bruker Optik GmbH, Ettlingen, Germany, pages 79 to 81 (2010), locking of an ATR crystal holder is released by means of a rocker switch and the holder is manually adjusted between an upper position for an optical observation mode and a lower position for an IR-ATR measuring mode. The holder is thereby moved below a Cassegrain objective. Several axial locking positions below the upper position allow adjustment of different pressure stages for contact with the sample.
Manual change of the operating modes and of the pressure stages is complex and limits the possibilities for automated sample investigations. There are no practicable automated ATR objectives to date.
It is the underlying purpose of the present invention to provide a practical automated ATR objective for an IR microscope, in particular, for facilitating changing of the operating modes and different contact pressures of the ATR crystal with respect to a sample.